A=int(input('输入AQI值：'))
if A<=50:
    print('优')
elif A>50 and A<=100:
    print('良')
elif A>100 and A<=150:
    print('轻度污染')
elif A>150 and A<=200:
    print('中度污染')
elif A>200 and A<=300:
    print('重度污染')
else:
    print('严重污染')
